Process of shell molding



Unite States Patent 3,004,312 PROCESS OF SHELL MOLDING Charles F. Froberger, Oak Park, Mich, assignor to General Motors Corporation, Detroit, Micln, a corporation of Delaware 7 t s No Drawing. Filed June 25, 1953, Ser. No. 364,202

4 Claims. (Cl. 22-193) This invention relates to improvements in shell molding and more particularly to an improved method of forming shell molds for use in the precision casting of metals.

Recently developed techniques in foundry practice employ thin-Walled dispensable molds and cores composed of sand and thermosetting binders. These procedures, generally referred to as shell molding processes, are particularly suited for the production of pre cision castings of a wide variety of metals.

, Essentially the shell molding process consists of employing a heat-hardenable plastic or' resin as a binder for the sand grains to form rigid molds having high gas permeability, good surface smoothness and dimensional stability. The molding material, generally a dry mixture of a major proportion of silica sand and a minor proportion of a thermosetting binder, is used in a powdered form with no water being added. Phenol-formaldehyde, phenol-furfural, melamine-formaldehyde and urea-formaldehyde resins are typical examples of the type of thermosetting binders preferably used. It is desirable that the sand employed be free of metal'oxides, clay, moisture and organic matter.

These sand-resin molds are prepared by allowing the dry mixture of sand and powdered resin to come into contact with a hot metal pattern for a short period of time. A layer of the mix adheres to the metal pattern surface due to the heating of the resin which entraps the sand with which it is intimately mixed, thereby accurately reproducing pattern details. The half patterns, gates and runners are usually all permanently fixed on metal plates. Metal patterns must be used because they are subjected to elevated temperatures. Pattern temperatures in the range between 250 F. and 450 F. are typical, but under certain conditions temperatures up to about 800 F. may be advantageously employed. The pattern temperature and the length of time the molding material is allowed to remain in contact with the hot pattern surface determine the resulting thickness of the mold. Mold build-up times generally range from a few seconds to approximately two minutes, depending upon the particular application.

After this short time interval, any excess or unbonded sand and resin are removed, and the closely adhering sandresin layer is cured, preferably by heating to a temperature within the range of about 300 F. to 1500" F. for a short period of time, usually from a few seconds to approvimately five minutes, while in contact with the metal pattern. This baking operation results in the conversion of-the resinous material to a hard, insoluble binder which securely bonds the sand grains together. After the removal of the pattern and mold from the curing oven, the mold is stripped from the pattern.- The completed molds are, in elfect, thin shells which posses sufiicient strength and stiffness to make them suitable for many casting operations. 1

- One of the primary reasons for the relatively restricted use of the shell molding process up to the present time has been the comparatively high cost of the organic binder. Inasmuch as the sand-resinmix normally contains up to about by weight of a thermosetting resin binder, the cost of the binder has been a substantial item of expanse when this process is employed. This is particularly true in applications which require the inclusion of a finely comminuted refractory material, such 3,004,312 Cg Patented Oct. 17, 1961 suitable refractory filler material, and the dry mixture,

is applied to the hot pattern without further prepara-' tion. Although the use of such a molding mix generally provides satisfactory shell molds in most respects, it also results in an undesirable residue build-up on the patterne Moreover, the shell molds heretofore used generally have proved to be somewhat brittle and not sufliciently strong or resilient to provide optimum results in all instances.

Accordingly, a principal object of the present inven tion is to provide an improved shell mold and a method. of preparing the same in which forming of the mold shell does not result in a pattern build-up. The elimina--- tion of this build-up in turn permits the same pattern to be used over a greatly extended period of time without: the necessity of removing the residue therefrom in order to accurately reproduce pattern details.

A further object of my invention is to provide an im-- proved shell mold and organic binder therefor which are appreciably less expensive than those generally here tofore used. A still further object of this invention is to provide a process for forming shell molds in which the amount of the relatively costly thermosetting resin binder may be substantially reduced, amounts in the order of approximately 2% by weight being all that is necessary for many applications.

The above and other objects are attained in accordance with my invention by the use of sand or other suitable comminuted refractory material having a substantial portion of its particles coated with an improved binder to form relatively inexpensive shell molds without building up a residue layer on the patterns. The improved organic binder which is usedto coat the sand particles comprises a mixture of novolak and a thermoplastic resinous material derived from wood. A small amount of a curing agent is also added in the molding mix to promote the proper setting of the binder upon subsequent contact of the molding mix with the hot pattern.

Certain of the resoles may be included in the binder as a substitute for a portion of all of the novolak provided they may be conveniently melted without setting. Generally, however, it is preferred to use only a small percentage of resole in place of novolak, and excellent results appear to be obtained when resole is omitted entirely. In all instances, of course, it is necessary that the binder mixture sets, at least partially, at the temperature of the hot pattern.

The thermoplastic resinous material derived from Wood may be blended with the novolak either by means During the formation of a shell mold according to.

the present invention, an improvedcasting-defining mold surface results from the co-condensation or copolymerization of the thermoplastic resinous material and the novolak to form a substantially completely thermosetproduct with a relatively small amount of expensive binder constituents. The novolak preferably is partially polymerized when mixed with the thermoplastic resinous material so that only a short baking period is required to cure the binder mixture.

It will be understood that the term mold, as used er n: s n ied n i generic.. ensetq ean a asti cnn.:- ,qh., na udes .bc h mo ds. nd cores; th s. nv t n s m nne ein m ted-. th rm n. m lar v.

nless-other se.indic ted he d p r s sederei a in ludin bothmqldp t msan be e .ihPI, je nd... dv n a f. h n ion l.

more fully appear -fron 1 the followingdetailed descriptionof preferredmethgds of employing my novel-binder o a m n xp sive e lmq which m a h qrmat c of. ea ade able es ue d-7 p on p ter s rf s-.1

In. preparing .theimproved organie binder for the shell' curing of the prineipal bi nder constituents when the ma ae m xi a e -placed into contact ht e t P iWmt.

' mia lv ecut naents quld n be P s i amount greater than approximately 25% by weight of the toft'al binder. It an fexcessof curing agent is emplay d e ma t e se ra -t p in e mold 011 P l a of t e when astin me l; a d. cracking of the me d cc rs. Hen eit ..:P l to d 'i n ina nt. n; a. moun .1 et ee p r x mat l v nd tth htotthabi s e f I hevefqundjt'hat, in order to properly benefit from he p e i vent qn. the l lit rtvp resin; h u d a e tute at least approximately 25% by weight of the binder.

Greater amount-gin some cases up to about 80% by Weight, l amav a ad-I. athe' orme o of s e mol s s i a le ermaax as na p r t as I h felled hat upe io es lts are btained he e n bin containsiabout 40 I to 60% by weight of the lignin-type aiaearr aim lr59% y. e e tb na e e at pre e t,

The terrnftherrrroplas tie material derived from wood, hereinafter used as descriptive of the thermoplastic materialin accordancewith the invention, is intended to include various wood derivative resinous materials in which the molecules contain both'methoxyl and hydroxyl groups. Such materials may be obtained fr m a variety of wood products includingsawdust, Wood chips, etc., by physi cal and/or chemieal treatment. Illustrative of one type ofthermoplastic resinous material isa substantiallygaso line-insoluble resinous--material obtained by extracting a resinous woodwith' a coal tar hydrocarbon, removing said hydrocarbonby evaporation, leaving a residue comprising a mixture of wood rosin and thermoplastic resinous material, extracting the rosin with a'petroleum hydrocarbon, leaving a thermoplastic resinous material, hereinafter referred to as a gasoline-insoluble wood resin.

Such a material is currently available commercially as- Vinsol" resin, aproduct of the Hercules Powder Company.

In practice,.. the amountof 'the-.organic binding -mix-.

of the molding mix. Hence, in order to obtain satisfactory casting'at a reasonable cost, a binder content between 3% and 7% is preferred.

, Ordinary sand orother comminuted refractory filler materials, such as zirconium silicate sands and flours, are appropriate fqruee, in practicing the present invention and normally-constitute. between and 9 8% of the;

weight-of themolding mix. In most cases it is desirable to employ a refractory material having an American Foundryma ns Soeiety fineness number within the range of about 25 to 180. At present I prefer to employ a refractorymaterial having an AFSfineness number within the range of 50 to 12 5.

In many instanees it isdesirableto also employ fine refractory fmaterialslorpowders, such as silica flour or powderedspent catalyst, etc. as fines to provide a better mold facing surface; In practice, amounts ofsuch facing materials within the rangeof about 2% to 20% byweight are used in many cases to provide a more heatresistant mold surface Thus-I have obtained very benefieial results in the casting ofsteel by a shell molding process when the mixture forming the shell mold is com;

c edfist nt ally ne e y g t dry sand, about 2% vto- 20% by weight of silica flour or other finely divided refractory materiahand about 5% to 15 of the, organic binder.

Inthe formation of a shell mold of the present type,

which is characterized by the elimination of pattern build-pp andexcellent casting-defining surfaces, the novolak may be first dissolved in a suitable solvent, such as methyl-ethylketone or a mixtureof toluene and methyl alcohol." After thenovolak is dissolved in the solvent, the thermoplastic material derived from wood is added to;the solutionand also dissolved therein. Normally as much of these constituents are dissolved in a In practice, when asufiicient amount of the thermoplastic material derived given ameunt of solvent as feasible.

from wood has been addedso that this resin and the npvolek eonstitute at least approximately 80% by weight of thetotal mixture or solution, the blend of these constituents is heatedtoa temperature at which it flows freely before it is mixed with the sand. Generally a temperature of at least 150 F. is therefore desirable. 0f course, the ,solvent and the binder constituents may be initially heated and maintained at this elevated tem perature while the latterare being dissolved in the solvent, a

The sand-is also preferably heated to prevent the organic binder constituents from cooling too rapidly and to insure the proper coating of the binder on the sand grains. Hence, I have-found itadvisable to heat the sand to a temperature of approximately 600 F. and then to permi t the sand to cool to approximately 400 F. be-

fore .the binder-solvent is introduced. In general, satis-. factory results are 'providedif the temperature of the sand is between 300 F. and 500 F. when the binder is added. During the mulling operation in which the bind-p er is completely blended with the sand, the mixture cools and the solvent; evaporates, leaving a' coating of the mixture of thenovolalgand the lignin-type resin on the sand particles.

Whenthe temperature of the coated sand has dropped to;a .temp eratur e which is sufliciently low to preclude setting or curing of the binder upon addition of the curing agent, the latter isadded to the molding mix. I

have found a temperature between approximately F, and' ISHO" F.'to be satisfactory in this respect. Likewise, the molding mix may be easily mulled at this temperature. The mulling'operation is continued, of course, until the curing agent is thoroughly mixed with the coated sand -grains.-

Although excellent results are obtained using the above described method of coating the sand particles with the novolak and thermoplatsic material derived from wood, it is not necessary tointroduce the. binder to the sand by means of a solvent. If the sand is maintained at a sufficiently high temperature, it is possible to add these powdered materials directly to the hot sand and to use the heat in the sand to melt and distribute the binder. The fiow characteristics of the lignin-type resin and ncvolak mixture are such as to permit satisfactory coating of the sand particles under these conditions. For example, the solvent may be omitted and the novolak and thermoplastic material derived from wood first can be melted together, or'separately melted and blended. Then this binder mixture may be permitted to cool and crushed to a powder form before being added directly to the hot sand. Likewise, it is also possible to separately or simultaneously mix the thermoplastic material derived from wood and novolak with the hot sand in powder form, or these two powders may be pre-mixed and then thoroughly blended with the sand which is preferably maintained at the aforementioned elevated temperatures. Alternatively, the novolak and the thermoplastic material derived from wood can be melted and added, either simultaneously or succesively to the heated sand while in a liquid condition. If desired, of course, these liquid binder constituents may be blended together, either as liquids or powders, before being introduced to the sand as a hot liouid.

After the particles of sand or other refractory material are coated with the organic binder in the above-describcd manner, the resulting mixture is placed in contact with the surface of a heated pattern having a temperature of at least approximately 350 F. At temperatures below about 350 F. the thermoplastic material derived from wood will not melt or flow to a sufficient extent to be of any value in strengthening the mold.

To obtain the proper resin flow, the mold mixture should remain in contact with the heated pattern surface for at least seconds with a contact time of between 20 and 35 seconds at the above pattern temperature usually providing highly satisfactory results. In general, there is no maximum pattern contacting time because the mold build-up on the pattern is sufiiciently rapid so that heat is quickly conducted from the pattern and soon results in the lowering of the pattern temperature below 350 P.

if no further heat is applied to this mold, thus precluding further flowing of the thermoplastic material or burning of the mold. In practice, however, approximately two minutes should be considered as the maximum period the mold should be allowed to contact the hot pattern before removal of the mold-pattern assembly for curing.

The resulting mold must be subsequently baked to complete the cure of the shell mold. The time of curing 'or baking in most cases should be between about 30 and 120 seconds, a 40 to 90 second cure normally providing optimum results. Of course, the length of the curing period depends on the baking temperature, the higher the curing temperature, the shorter the curing time, and vice versa. Curing temperatures between 350 F. and 1500 F. have proved to be satisfactory although in some cases oven temperatures as high as 1800 F. may be employed. Placing the molded mix into contact with the hot metallic pattern, together with the subsequent baking operation, cures or hardens the binder by causing a copolymerization of the lignin-type resin and the thermosetting resin with the aid of the included small proportion of hexamethylenetetramine, paraformaldehyde or similar curing agent. The sand-resin shell-type mold thus produced has excellent permeability, possesses high transverse strength and exhibits no tendency to cause blowing on the casting when the molten metal is poured.

Shell molds formed in accordance with the aboveoutlined procedure, while utilizing a relatively small amount of expensive thermosetting resin, thus appreciably reducing the cost, can be employed in the casting of metals to very close dimensional tolerances since the casting-defining surfaces of the mold, while readily permeable to gases, have high strength and smoothness.

These molds have proved to be highly satisfactory at! casting metals poured at temperatures as high' as 2800 F. and are therefore especially adapted for pouring steel. As indicated above, it is usually desirable to include a;

minor proportion of a fine refractory filler when casting high-temperature ferrous-base metals. above method of forming inexpensive high-strength shell molds permits the precision casting of non-ferrous as well and the molds produced thereby is the'fact that these.

molds offer very little resistance to the expansion and contraction of the molten metal subsequent to pouring, thus minimizing the danger of formation of cracks or hot tears. The cured shell molds also accurately reproduce pattern details, have no affinity for water, are stable under atmospheric conditions, and may be stored indefinitely. Furthermore, these molds can be produced and processed without dust formation. Moreover, the resultant castings have unusually smooth and clean surfaces, true dimensions, and a minimum of fin at the parting line. The surfaces of these castings are free of residual mold material, thereby eliminating the necessity of shot blasting. This process and the molds formed in accordance therewith can be used to provide castings of extremely thin section due to the unusual smoothness and high gas permeability of the molds. g

It will be understood, of course, that various changes and modifications of the embodiments of the invention described herein may be made by those skilled in the art without departing from the spirit and principles of the invention, the scope of which is defined in the following claims.

I claim:

1. A process for forming a smooth-surfaced shell mold without building up a residue layer on a pattern, said process comprising thoroughly mixing a heated comminuted refractory filler with a mold binder comprising novolak and a thermoplastic resinous material obtained by extracting a resinous wood with a coal tar hydrocarbon, removing said hydrocarbon by evaporation, leaving a residue comprising wood rosin and thermoplastic resinous material, and extracting the rosin from said residue with a petroleum hydrocarbon, permitting said sand to cool to thereby cause a coating of the novolak and thermoplastic resinous material to be formed on a substantial portion of the particles of refractory filler, thereafter mixing a small but effective amount of a binder curing agent with the coated sand particles, placing said coated sand particles into contact with a metallic pattern heated to a temperature between 250 F. and 800 F. for at least ten seconds, and thereafter curing the resultant shell mold while in contact with said pattern.

2. A method of forming a molding mixture to be applied to a hot pattern for forming a shell mold, said method including thoroughly mixing a heated comminuted refractory filler with a mold binder comprising novolak and a thermoplastic resinous material derived from wood containing both methoxyl and hydroxyl groups, permitting said sand to cool to thereby cause a coating of the novolak and thermoplastic resinous material to be formed on thepartlcles of refractory filler, and thereafter mixing a small but effective amount of a binder curing agent with the refractory filler and binder.

3. A method of forming a molding mixture to be applied to a hot pattern for forming a shell mold, said method comprising dissolving in a suitable solvent novo- Of course, the

The gases which are generated.

7 an le: newin o uble thermoplastic w.-oed; resin; containing o e o yl nsl'hy o y'l group ntima e yv mi dn 11 c resultant; solution with; heatfidisand, thereafter permitting;saidi'solvent toevaporateso-as toleave a;

cgatingmfihe m-ixture;of.-;the thermoplastic, wood resin and; novolak; on. the sandgrains,- subsequently adding a;

small-:buteffective amount oia'bindencuringagent to the, coatedsand grains, and finallyrmulling the formed mix: ture until the curing agent is thoroughlyrnixed withthe, coated sand grains:

4..-z Therprocess of forming a molding hmixturegto be applied to a hot pattern for .formingaashell mold, said process comprising heating sand; to atemperature of at least: 300 'Fi, melting novolak iand a gasolinerinsolnble resinousmaterial obtained by extracting a resinous. wood with a coal tar hydrocarbon, removing said hydrocafbor v xc t pp oximately and thereafter thoraoughly mixing a binder curing agentwiththe co atedsandv particles inanamount equal to about 8% to 15% of the.

total weight of the noyolalglhermoplastic:resinous mate-. rial and curing agent. a

References. fiitediinithe file. of this. patent.

UNITED, STATESYPATENTS 2,101,330 Delaney Dec. 7, 1937 2,405,650 Hartwig Aug, 13, 1946 2,444,413 Weston r v July,6, 1948 2,491,006 Grube, h r r Dec. 13, 1949 2,549,822 Koonce Apr. 24,1951 2,630,608; Granathv "Mar. 10,1953 2,683,296 Drummet ah- ,July 13, 1954 2,706,188 Eitkov et al., Apr. 12,1955 2,751,650, Froberger.; 1 1 1.Junee215, 19

OTHER [REFERENCES Am, Foundryman, August, 1952, pages 42-;56.-- The Fgundryman, October;;19 5,0, paga -162, 164 and 16,8.

Steel, December}, 1951, pages ,90and 92.1 

1. A PROCESS FOR FORMING A SMOOTH-SURFACED SHELL MOLD WITHOUT BUILDING UP A RESIDUE LAYER ON A PATTERN, SAID PROCESS COMPRISING THOROUGHLY MIXING A HEATED COMMINUTED REFRACTORY FILLER WITH A MOLD BINDER COMPRISING NOVOLAK AND A THERMOPLASTIC RESINOUS MATERIAL OBTAINED BY EXTRACTING A RESINOUS WOOD WITH A COAL TAR HYDROCARBON, REMOVING SAID HYDROCARBON BY EVAPORATION, LEAVING A RESIDUE COMPRISING WOOD ROSIN AND THERMOPLASTIC RESINOUS MATERIAL, AND EXTRACTING THE ROSIN FROM SAID RESIDUE WITH A PETROLEUM HYDROCARBON, PERMITTING SAID SAND TO COOL TO THEREBY CAUSE A COATING OF THE NOVOLAK AND THERMOPLASTIC RESINOUS MATERIAL TO BE FORMED ON A SUBSTANTIAL PORTION OF THE PARTICLES OF REFRACTORY FILLER, THEREAFTER MIXING A SMALL BUT EFFECTIVE AMOUNT OF A BINDER CURING AGENT WITH THE COATED SAND PARTICLES, PLACING SAID COATED SAND PARTICLES INTO CONTACT WITH A METALLIC PATTERN HEATED TO A TEMPERATURE BETWEEN 250*F. AND 800*F. FOR AT LEAST TEN SECONDS, AND THEREAFTER CURING THE RESULTANT SHELL MOLD WHILE IN CONTACT WITH SAID PATTERN. 